Also, peak users will know their stations (mostly)

I want it for when I'm going to meetings, using stations I'm not familiar with. What normally happens is I leave too late (just one more email …), then as I change tube I have to walk the entire length of the platform to exit, arriving at the next platform just as a train leaves …

Saving me a couple of minutes on that journey would be worthwhile.

In a fully loaded Tube train, at peak hour, there will be people boarding and leaving in more or less equal numbers at Zone 1 stations.

iPhone/iPod Touch penetration of tuberiders? er…5%?

Full train = 1000 people

iPhone/iPod Touch users per train = 50

% of them with the app, once word gets around: er, 25%?

Number of people who can comfortably stand near the doors that open at the exit position, ready to sprint out and get ahead of others exiting further down? – er…15?

And they can't ALL be app users by that door. There will be an average distribution of normal folk.

So by my verrrrry hairy maths, it won't take too many people using the app at peak times to render it not that useful anymore.

It would be interesting to see LU's figures on the distribution between exit stations though.

but

The classic 'Tragedy of the commons' played out on mobile.

The early adopters will have a great time. Then, you'll notice more and more iPhone users in the same carriage. Then non-iPhone users will recognise the signs and follow the ones with the iPhones out.

I give it 6 months before the carriages near the exits are so rammed you can't breathe

Then the developer will launch an app to track the carriages that *aren't* near the exits.